1. Field of Invention
This invention relates to a phased-array receiver which is used in an ultrasonic diagnostic apparatus of the like, for suitably varying the phases of the voltages from transducer elements that receive ultrasonic waves, and for summing these voltages to change the directional or other characteristics or ultrasonic waves.
2. Description of the Prior Art
Electronically scanned ultrasonic diagnostic apparatuses can be classified into two categories or types according to the principles on which they operate. One type is a linear scan. Another type is a sector scan.
In a linear scan, an array of transducer elements transmits or receives ultrasonic beams directed substantially perpendicularly to the array. The transducer elements are successively excited as if one ultrasonic beam were translated for scanning purposes. Therefore, according to this type of scan, the scanning lines generated parallel to each other produce a tomogram of an object under examination.
In a sector scan, almost all of the transducer elements of an array are excited in different timed relationships, to electronically control the directions of propagation of the transmitted or received ultrasonic beams, and thereby perform a scan in the shape of a geometrical sector.
According to these different principles of operation of both types of scanning, ultrasonic diagnostic apparatuses of linear scan systems are generally used to diagnose a human abdomen, and sector scan systems are generally used to observe the interior of a human body through space between neighboring ribs, thereby to diagnose the heart or to accomplish other similar purposes.
In this manner, ultrasonic diagnostic apparatuses of linear and sector scan systems were once manufactured and operated as entirely different apparatuses.
However, in recent years, an apparatus which is functionally either a cross between the apparatuses of the mentioned two types of scans, or a device belonging to neither type, has been devised. Furthermore, ultrasonic diagnostic apparatuses have been required to perform both linear and sector scans using the same apparatus.
An apparatus, which satisfies most of the foregoing requirements, is depicted in FIG. 1, in a general block diagram. The apparatus has a delay line 1. The delay line 1 may comprise, for example, the components depicted in FIG. 3, where each LC unit circuit comprises an inductor L and a capacitor C, provided between neighboring taps a.sub.1 -a.sub.n, to delay input electrical signal applied thereto by equal increments of time. In this specification, a delay line means a delay line equipped with a plurality of taps, such as depicted in FIG. 3.
The system of FIG. 1 further includes a crosspoint switch 3, wherein switching elements are placed in a matrix, that is at points where longitudinal lines (output lines) and lateral lines (input lines) meet. These switching elements are opened or closed under control of a controller 4. A plurality of transducers, each of which converts electrical input signals to acoustic output signals, constitute an array A.sub.i. Each transducer element is made, for example, of lead zirconate titanate (PZT). For sake of clarity, only four transducer elements are depcited in the array of FIG. 1. However, in a practical embodiment, a large number of transducer elements, for example, 32, may be provided. Also shown are a connector B and a set of amplifiers U.sub.i. The outputs of amplifiers U.sub.i are connected to the lateral lines of the crosspoint switch 3. The output signal from delay line 1 is amplified by an amplifier G and supplied for use.
Although the circuit of FIG. 1 is represented as a configuration for processing signals received by array of transducer elements A.sub.i, if amplifiers U.sub.i and G are bidirectional devices, the circuit can also serve to transmit ultrasonic beams from transducer elements A.sub.i. Thus, circuits discussed herein can serve both transmission and reception functions. In the instant discussion, however, the components will be treated generally as receiving circuits for sake of convenience and simplicity.
If delay line 1 of FIG. 1 has a sufficiently high resolution of delay time and a sufficiently large number of taps of the delay line to accommodate both linear scan and sector scan, and if the number of switching elements in crosspoint switch 3 is sufficiently large, the receiver shown in FIG. 1, can perform both linear and sector scans. The receiver may reasonably be said to be a general purpose apparatus because all (four in FIG. 1) of the transducer elements needed to obtain a visual image can be connected to crosspoint switch 3, and because delay line 1 has a sufficiently high resolution of delay time and a sufficiently large number of taps to achieve different types of scans ranging from linear scan to sector scan. Controller 4 in such cases is used to close and open selectively any desired crosspoint in the matrix switch 3 when performing the different type of scan.
FIG. 1 shows a configuration where the array of transducer elements A.sub.i are connected for sector scan. Thus, operation in a sector scan mode will be briefly described, with reference to FIG. 6. In FIG. 6, the manner of propagation of reflected ultrasonic waves, or echoes, is shown. As an example, an echo from point P.sub.1 returns to array A.sub.i in the form of a wavefront shown in FIG. 6. Thus, the times at which the echo reaches the transducer elements differ according to the incident angle of the echo to the transducer element. Thus, in order to exactly detect the echo from point P.sub.1, for example, suitable time delays are given to electrical signals which are produced from the transducer elements in response to arrival of the echo so that the electrical signals may be equivalently in phase. Since the system of FIG. 1 is equipped with an ideal delay line 1 and crosspoint switch 3, phase adjustment can be readily made as mentioned above.
When a linear scan is made by the system of FIG. 1, the probe for sector scan is replaced by a probe comprising an array of transducer elements A.sub.i such as depicted in FIG. 2, and connection of the latter probe is made via connector B. Generally, probes for linear scan comprise a large number of transducer elements than probes for sector scan. However, the numer of transducer elements excited for a single ultrasonic beam is equal to or less than that of sector scan. In FIG. 2, four transducer elements are excited irrespective of the type of scanning mode. These excited transducer elements are successively selected for every ultrasonic scanning beam by selector circuit 6 in the probe as shown in FIG. 2. Consequently, the receiver shown in FIG. 1 receives substantially the same number of input signals through connector B, whether the scan is made linearly or in sectorial manner. Thus, the receiver can be used for both types of scanning modes.
In linear scan, an echo enters the array of transducer elements A.sub.i at angles smaller than those in case of sector scan. Therefore, delay line 1 and crosspoint switch 3 of FIG. 1 can suitably perform linear scan. As hereinbefore described, the system of FIG. 1 can perform linear scan to sector scan, but the receiver has certain deficiencies.
For example, when a sector scan is executed by the system of FIG. 1, efficient use is made of delay line 1 and cross point switch 3. However, when the system is used for linear scan, only a small part of the delay line 1 and crosspoint switch 3 are utilized. Hence, the system of FIG. 1 is, for such purpose, redundant.
Moreover, since delay line 1 has only a single output, it is impossible to simultaneously obtain a plurality of azimuth angles or electrical echo signals emanating from a plurality of acoustic positions.
Also, only one switching element is allowed to be closed at one time for each lateral (i.e. input) line of crosspoint switch 3, as may be understood by study of the structure of delay line depicted in FIG. 3. Thus, delay line 1 operates at a low time-wise efficiency.